In 1991, the 25-year-old construction worker with a drink problem and a tendency to drive too fast was severely beaten by several white Los Angeles police officers. The attack was caught on videotape, the beating was seen round the world and overnight King became the poster boy for black victims of police brutality in the post-civil rights era. At a moment in American history when the charred memories of fire hoses, German shepherds and viciously slain black teenagers, like Emmett Till, were supposed to fade away, new painful memories were ignited.

Such horrors were not supposed to happen to black people outside the south and certainly not after the United States had become a "more perfect union" with the passage of the 1964 Civil Rights Act. King was not born in Birmingham, Alabama, in the midst of Jim Crow. He was born in Sacramento, California, in 1965; he grew up just outside Los Angeles, an hour's drive from Disneyland.

In post-civil rights America – just as in theme parks from California to Florida – dreams were supposed to come true, a black boy was supposed to have a chance. Racism had not only been reimagined as a thing of the past; it had been whitewashed from the cultural landscape.

Looking back over the past two decades through the lens of King's life – including the acquittal and mistrial of the four officers (two were later convicted of civil rights violations in federal court) and the 1992 LA uprising that followed – reveals that police intimidation and/or violence has become a normal rite of passage for millions of black people in America today. Since 2006, in New York City alone, roughly a half million black and Latino people every year are treated as would-be criminals through a perfectly legal policy of racial profiling known as stop, question and frisk. Required to prove their right to pass freely in the financial and cultural capital of the free world, those stopped are almost all innocent of any criminal offence.

What happened to King on 3 March, 1991, was indeed a tipping point. The increased scale and intensity of police harassment or brutality outside the south – in places like New York, Chicago or Los Angeles – was laid bare. In an interview with me about his book, The Riot Within, King joked about how common police violence was in his old neighbourhood. He described well-known cop bullies who bashed the heads of his teenage friends into the sides of squad cars, then showed off the dents to intimidate other kids.

The videotape of King's racially charged beating created a crisis of legitimacy that rippled across every police agency in America, and eventually toppled Daryl Gates, the LAPD police chief who was untouchable until King. The videotape brought into plain view what had been going on for decades since black people had left the post-reconstruction south. In a 1929 Illinois Crime Survey, for example, researchers found that African-Americans made up 30% of the recorded police killings but only 5% of the population.

Before 1965, few observers challenged the conventional wisdom that northern anti-black police violence was a vestige of old southern racism – imported from Birmingham, not homegrown. After 1965 in the wake of the Watts Riot and the long hot summer of 1968 following Martin Luther King's assassination, the conventional wisdom for police brutality was that it was the legitimate reaction to pathological behaviour stemming from fatherless households, juvenile delinquency and sub-cultures of crime and drugs.

With the exception of what was televised during the riots, police violence had remained covert and invisible outside poor black communities.

The crisis of legitimacy and sharp break with the past came with the King videotape. The raw violence did not match three decades of post-racial mythology, where the only racism supposedly left in America was affirmative action. King's tape and others that followed, along with greater federal scrutiny of police agencies, compelled criminal justice officials and politicians to re-engineer or rig the system into a labyrinth of ostensibly race-neutral policies – such as stop, question, and frisk or stand your ground laws. It is now this system that legitimates the routine targeting of young black men for harassment or violence and that is now under attack as witnessed by yesterday's silent march in New York City.

Three years ago, before his memoir was published, King gave an interview to a reporter where he claimed his place in history. "It would be a real mistake on my part," he said, "if I didn't find my spot in history before I leave this earth." He achieved his goal.